in sapien, i try to run radiosity, but it slows down alot and only shows purple mist

depending on the speed of your computer, radiosity can take quite a bit of time.
I would suggest leaving it alone, then after all the numbers in the radiosity countdown reach 0, type in radiosity_save after you hit tilde (~) in the sapien window.

i did that, now i just have purple mist and the box used for frame in modeling

I got something like that in the tutorial scenario when I tried to rerun radiosity with a different sky. I was aiming to get some more light, since the default lighting is pretty dark. It turned the map an ugly shade of brown and had horrid shadows. I'm not sure what the error is.

That is why you always run radiosity in tool before the final pass.

Open tool, type:
tool lightmaps levels\LEVELNAME\LEVELNAME LEVELNAME 0 0.9

Then later, before you release the map, run it in sapien

...saves a massive amount of time.

Trust me, I know.

are you sure you applied the +sky material to the part of your level that you want to be sky?